var xPos = 20;

var yPos = document.body.clientHeight;

var step = 1;

var delay = 30;

var height = 0;

var Hoffset = 0;

var Woffset = 0;

var yon = 0;

var xon = 0;

var pause = true;

var interval;

img.style.top = yPos;

function changePos() {

width = document.body.clientWidth;

height = document.body.clientHeight;

Hoffset = img.offsetHeight;

Woffset = img.offsetWidth;

img.style.left = xPos + document.body.scrollLeft;

img.style.top = yPos + document.body.scrollTop;

if (yon) {

yPos = yPos + step;

}

else {

yPos = yPos - step;

}

if (yPos < 0) {

yon = 1;

yPos = 0;

}

if (yPos >= (height - Hoffset)) {

yon = 0;

yPos = (height - Hoffset);

}

if (xon) {

xPos = xPos + step;

}

else {

xPos = xPos - step;

}

if (xPos < 0) {

xon = 1;

xPos = 0;

}

if (xPos >= (width - Woffset)) {

xon = 0;

xPos = (width - Woffset);

}

}

function start() {

img.visibility = "visible";

interval = setInterval('changePos()', delay);

}